Inkjet printers operate a plurality of inkjets in each printhead to eject liquid ink onto an image receiving member. The ink can be stored in reservoirs that are located within cartridges installed in the printer. Such ink can be aqueous ink or an ink emulsion. Other inkjet printers receive ink in a solid form and then melt the solid ink to generate liquid ink for ejection onto the image receiving surface. In these solid ink printers, also known as phase change inkjet printers, the solid ink can be in the form of pellets, ink sticks, granules, pastilles, or other shapes. The solid ink pellets or ink sticks are typically placed in an ink loader and delivered through a feed chute or channel to a melting device, which melts the solid ink. The melted ink is then collected in a reservoir and supplied to one or more printheads through a conduit or the like. Other inkjet printers use gel ink. Gel ink is provided in gelatinous form, which is heated to a predetermined temperature to alter the viscosity of the ink so the ink is suitable for ejection by a printhead. Once the melted solid ink or the gel ink is ejected onto the image receiving member, the ink returns to a solid, but malleable form, in the case of melted solid ink, and to a gelatinous state, in the case of gel ink.
A typical inkjet printer uses one or more printheads with each printhead containing an array of individual nozzles through which drops of ink are ejected by inkjets across an open gap to an image receiving surface to form an ink image during printing. The image receiving surface can be the surface of a continuous web of recording media, a series of media sheets, or the surface of an image receiving member, which can be a rotating print drum or endless belt. In an inkjet printhead, individual piezoelectric, thermal, or acoustic actuators generate mechanical forces that expel ink through an aperture, usually called a nozzle, in a faceplate of the printhead. The actuators expel an ink drop in response to an electrical signal, sometimes called a firing signal. The magnitude, or voltage level, of the firing signals affects the amount of ink ejected in an ink drop. The firing signal is generated by a printhead controller with reference to image data. A print engine in an inkjet printer processes the image data to identify the inkjets in the printheads of the printer that are operated to eject a pattern of ink drops at particular locations on the image receiving surface to form an ink image corresponding to the image data. The locations where the ink drops landed are sometimes called “ink drop locations,” “ink drop positions,” or “pixels.” Thus, a printing operation can be viewed as the placement of ink drops on an image receiving surface with reference to electronic image data.
Phase change inkjet printers form images using either a direct or an offset print process. In a direct print process, melted ink is jetted directly onto recording media to form images. In an offset print process, also referred to as an indirect print process, melted ink is jetted onto a surface of a rotating member such as the surface of a rotating drum, belt, or band. Recording media are moved proximate the surface of the rotating member in synchronization with the ink images formed on the surface. The recording media are then pressed against the surface of the rotating member as the media passes through a nip formed between the rotating member and a transfix roller. The ink images are transferred and affixed to the recording media by the pressure in the nip. This process of transferring an image to the media is known as a “transfix” process. The movement of the image media into the nip is synchronized with the movement of the image on the image receiving member so the image is appropriately aligned with and fits within the boundaries of the image media.
When the image receiving member is in the form of a rotating drum, the drum is typically heated to improve compatibility of the rotating drum with the inks deposited on the drum. The rotating drum can be, for example, an anodized and etched aluminum drum. A heater including a heater reflector or housing can be mounted axially within the drum and extends substantially from one end of the drum to the other end of the drum. A heater unit includes one or more heating elements located within the heater reflector with each one being located approximately at each end of the reflector. The heater remains stationary as the drum rotates. Thus, the heaters apply heat to the inside of the drum as the drum moves past the heating elements backed by the reflector. The reflector helps direct the heat towards the inside surface of the drum. Each of the heating elements is operatively connected to a controller which is configured to control the amount of power applied to the heating elements for generating heat. The controller is also operatively connected to temperature sensors located near the outside surface of the drum. The controller selectively operates the heater to maintain the temperature of the outside surface within an operating range.
In one embodiment, the controller is configured to operate the heater in an effort to maintain the temperature at the outside surface of the drum in a range of about 55 degrees Celsius, plus or minus 5 degrees Celsius. The ink that is ejected onto the print drum has a temperature of approximately 110 to approximately 120 degrees Celsius. Thus, images having areas that are densely pixelated, can impart a substantive amount of heat to a portion of the print drum. Additionally, the drum experiences convective heat losses as the exposed surface areas of the drum lose heat as the drum rapidly spins in the air about the heater. Also, contact of the recording media with the print drum affects the surface temperature of the drum. For example, paper placed in a supply tray has a temperature roughly equal to the temperature of the ambient air. As the paper is retrieved from the supply tray, it moves along a path towards the transfer nip. In some printers, this path includes a media pre-heater that raises the temperature of the media before it reaches the drum. These temperatures can be approximately 40 degrees Celsius. Thus, when the media enters the transfer nip, areas of the print drum having relatively few drops of ink on them are exposed to the cooler temperature of the media. Consequently, densely pixilated areas of the print drum are likely to increase in temperature, while more sparsely covered areas are likely to lose heat to the passing media. These differences in temperatures result in thermal gradients across the print drum.
Transfer defects can occur if the drum temperature exceeds about 62° C. When the thermistors measure a drum surface temperature of 57-58° C., the fan is turned on to start cooling the drum. When the thermistors measure a drum temperature that is too low, the heater is turned on until the thermistor measurements are within the control band of acceptable temperature. Hot ink jetted onto the drum surface increases the temperature of the drum in areas of high ink density. In areas without ink, the print media tends to cool the drum surface. Long printing jobs with prints containing areas of high ink density on one portion of the print and other areas of the print with little or no ink can create significant temperature differences between the ink and no ink locations on the drum. With temperature sensing only at the ends of the drum, detection of a temperature difference can be difficult if detected at all. If the temperature difference is detected, then a single fan and dual circuit heater can be incapable of correcting the temperature difference before image quality defects result. The thick walls of the drum can include a large mass of aluminum which cannot be rapidly heated or rapidly cooled. The large mass can help to prevent the generation of an image defect caused by temperature differences. If large temperature differences do occur, however, a reduction in the temperature difference can be made too slowly by the heater or fan to avoid defects.
Efforts have been made to control the thermal gradients across a print drum for the purpose of maintaining the surface temperature of the print drum within the operating range. Simply turning the heater on and off can be insufficient because the ejected ink can raise the surface temperature of the print drum above the operating range, even when an individual heating element is turned off. In some cases cooling is provided by adding a fan at one end of a print drum. The print drum is open at each flat end of the drum. To provide cooling, the fan is located outside the print drum and is oriented to blow air from the end of the drum at which the fan is located to the other end of the drum where it is exhausted. The fan is electrically operatively connected to the controller so the controller activates the fan in response to one of the temperature sensors detecting a temperature exceeding the operating range of the print drum. The air flow from the fan eventually cools the overheated portion of the print drum at which point the controller deactivates the fan.
While the fan system described above can generally maintain the temperature of the drum within an operating range, some inefficiencies do exist. Specifically, one inefficiency can arise when the surface area at the end of the print drum from which the air flow is exhausted has a higher temperature than the surface area near the end of the print drum at which the fan is mounted. In response to the detection of the higher temperature, the controller activates the fan. As the cooler air enters the drum, it absorbs heat from the area near the fan that is within the operating range. This cooling can result in the controller turning on the heater for that region to keep that area from falling below the operating range. Even though the air flow is heated by the region near the fan and/or the heating element in that area, the air flow can eventually cool the overheated area near the drum end from which the air flow is exhausted. Nevertheless, the energy spent warming the region near the fan and the additional time required to cool the overheated area with the warmed air flow from the fan adds to the operating cost of the printer. Thus, improvements to printers to heat and to cool a print drum are desirable.
The transfix solid ink printing process requires that the image drum surface be maintained within a relatively narrow temperature range. If the temperature is too low, the ink image will not spread under pressure in the transfix nip. If the temperature is too high, transfer from the image drum to print media will be poor. Conventional systems use a heater and a cooling fan to adjust the drum temperature based on thermistor temperature readings outside of the print area on the inboard and outboard ends of the drum. Drum temperature uniformity is influenced by media size, weight and mix, image density and distribution on the prints, and job length. Low area coverage prints cool the drum and high area coverage prints heat the drum in the location of the ink. The resulting temperature gradients on the drum surface can be large enough to generate local defects due to high or low temperatures. Thinner drums are desirable for cost and drive torque, but are more susceptible to temperature gradients due to lower mass. Thicker drums are less susceptible to temperature gradients, but also take longer to heat or cool due to higher mass. It is also desired to increase the diameter of the drum for production applications of solid ink jet printers to increase printer throughput. Larger drums, however, generally require thicker drums for mechanical strength which can increase the occurrence of temperature gradients. The temperature difference problems can also be more prevalent in larger systems used for printing many copies of the same documents, because many ink images can be expected to be the same or similar in long production jobs which can increase the likelihood of localized heating on the drum.